Add ai_catalog object storage configuration and feature flag

What does this MR do and why?

Adds infrastructure for AI Catalog object storage support:

• Add ai_catalog_yaml_object_storage feature flag (user-scoped, WIP)
• Add ai_catalog object storage config to gitlab.yml.example
• Register ai_catalog in object storage settings initializer
• Add stub helper for ai_catalog object storage in tests

This is Part 1b of a series splitting up !227895 to add object storage support for large YAML definitions in the AI Catalog.

MR Series

MR	Description	Dependencies
1a	Add yaml_definition_file columns to ai_catalog_... (!228120 - merged)	None
1b (this MR)	Add ai_catalog object storage configuration and... (!228122 - merged)	None
2	YamlDefinitionUploader (CarrierWave uploader)	1b
3	Model layer: Mount uploader, add transparent accessor	1a, 2
4	Service layer & schema: Upload in services, make yaml_definition optional	3

Issue: #591638

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.